This invention relates to a high volume, low cost process for preparing highly purified silicon suitable for use in fabricating silicon solar cells and other semiconductor devices. More particularly, this invention relates to a process for forming distillable binary silicon fluoride homologues which when heated yield semiconductor grade silicon and silicon tetrafluoride.
The electronics industry requires ever increasing amounts of semiconductor grade silicon, where "semiconductor grade silicon" means very high quality silicon of high purity, having negligible amounts of undesirable contaminants. This demand is further growing because of the rising interest in silicon solar cells for the production of electricity. While the high quality is a continuing requirement, the reduction in cost of this high quality silicon is also very important, especially for large area solar cells.
At present, most semiconductor grade silicon is produced by reacting impure metallurgical grade silicon with anhydrous hydrochloric acid to produce trichlorosilane. The trichlorosilane can be purified by distillation, and is then reacted with hydrogen to deposit pure polycrystalline silicon on a heated filament. This process is expensive, partly because it is not a continuous flow, automated process. Many of the steps presently used in this conventional silicon production process are incompatible with continuous process concepts, and thus modification or straightforward automation of the present process is unlikely to result in significant cost savings.
Alternate processes for the production of silicon, and especially for the production of semiconductor grade silicon, have been suggested. In Pease, U.S. Pat. No. 2,840,588, the idea was advanced for reacting silicon and silicon tetrafluoride to form silicon difluoride gas. This gas could be polymerized and then pyrolized to produce silicon. There was no suggestion or teaching in Pease that such a process could result in the large scale production of silicon, nor that the process could be modified to produce distillable binary silicon fluoride homologues.
In the above referenced Ingle application, the disclosure of which is incorporated by reference herein, the ideas of Pease are advanced and a process is disclosed for the continuous production of silicon. In that process the thermal conversion of the silicon difluoride polymer results in the production of appreciable quantities of volatile binary silicon fluorides in addition to silicon. These volatile binary silicon fluorides would distill out in the hot zone of the reaction apparatus and would not be converted into silicon. The resulting oily residues reduce silicon transport yields and hinder silicon removal because of their pyrophoric nature. The process was thus designed to force the reaction towards the production of silicon at the expense of the volatiles, but this lowers the overall efficiency for the production of silicon.
Thus, while there are a variety of ways for preparing semiconductor grade silicon, some well-established and others only developmental, there still remains a need for a high volume process which will produce pure silicon at a low cost.
Accordingly, it is an object of this invention to provide a process for the production of semiconductor grade silicon. The process is continuous, low cost, and can be highly automated.
It is a further object of this invention to provide a process for the production of binary silicon fluoride homologues which can subsequently be disproportionated to yield pure silicon and silicon tetrafluoride.
It is a still further object of this invention to provide a process for the purification of metallurgical grade silicon by heating that impure silicon in the presence of silicon tetrafluoride.